A. Field of the Invention
This invention relates to batches of raw materials for preparing glass and more specifically to a method for preparing pre reacted batches of raw materials for the production of glass formulas, to which were added cullet (bits of recycling glass) to increase the velocity of the calcination process, the decarbonization grade of the batch and the formation of the desirable cristaline structures.
B. Description of the Related Art
The batches for preparing molten glass have been provided, since many years, by feeding independent glass components typically silica, sodium carbonate, calcium carbonate, borates, feldspar, dolomite, kaolin, etc., in proportions according,to a desired glass formulation, to a melting furnace at temperatures ranging between 1400 to 1600° C.
These typical batches include raw materials having different melting points and reacting at different temperatures under different operating conditions.
During the melting process of the raw materials, many different reactions take place in the glass melting furnace, which producing gaseous emissions in the form of bubbles which create the need for establishing a refining and conditioning zone for the molten glass mass in the furnace which, in turn, results in a limitatior of the residence time of the molten mass, resulting in high melting temperatures and the need to carefully control environmental emission restrictions.
Additionally, because of the high temperatures and corrosive nature of the diverse types of reactions taking place in the melting furnace, the life of the melting furnace is always an important issue.
The U.S. Pat. No. 3,082,102 issued to Cole et al, discloses a process for producing molten glass by maintaining the glass batch at a temperature and for a time sufficient to complete chemical reaction between component particles while the batch as a whole remains in solid state at the completion of the reaction, before subjecting the embryo glass so formed to a temperature high enough to melt the embryo glass.
The U.S. Pat. No. 4,920,080 issued to Demarsest, discloses a method for pre-heating and pre-reacting all portions of the batch prior to the melting step, in two separate portions, a first portion of SiO2 with Na2CO3 in a first pre-reaction zone at sufficient time and temperature to form a product consisting predominantly of sodium silicate, and heating a second portion of SiO2 with CaCO3 in a second pre-reaction zone at sufficient time and temperature to render the calcium source free of carbonates.
It can be concluded from the methods disclosed in the above mentioned patents that efforts have been made to provide pre-reacted raw materials in which gaseous compounds have advantageously been eliminated.
However, the above disclosed methods treat all the batch mixtures at temperatures finely controlled to avoid that the reactions taking place do not form a liquid melting phase because of the danger representing the difficulty of handling a batch including solid and liquid phases.
In the U.S. Pat. No. 6,358,870 issued to Antonio Pita and Hugo Bolio, and its corresponding Mexican patent application No. NL/a/2001/000043, is mentioned that a batch for the different purposes, mainly for flat glass, container glass (soda-lime and boro-silicate glass), glass fiber, etc., consists of a molecular formula comprising a diverse number of molecules of Si, Na, Ca, Mg, Bo, etc., which can be clearly approximated from natural, substances, partially treated minerals or intermediate products of treated minerals, including molecular systems of Si—Na, Si—Na—Ca, Si—Na—Mg, Si—Ca—Mg, Si—Na—Ca—Mg and mixtures thereof, some of which are in the form of already pre-reacted substances and some of which have to be conveniently pre-reacted in a calcining burner and, in either case, they are substantially free from gaseous carbon dioxide.
Also is described that, if phase diagrams are prepared for the different molecular systems of raw materials, it is possible to select molecular formulas having decomposing and/or melting temperatures well above at least 1000° C., below which not only melting glass or liquid phase is not formed, but also the release of carbon dioxide can be clearly carried out, which are selected from invariant points or from points on a line connecting invariant points of phase diagrams of said molecular systems, and combine them to reach or approach the desired molecular glass formula, completing this by adding pure silica when necessary.
Also is commented that some of the mentioned of the phase diagrams of the above disclosed nature can be found for example in the papers of K. A. Shahid & F. P. Glosser “Phase equilibrium in the glass forming region of the system Na2O—CaO—MgO—SiO2” published in Physics and Chemistry of Glasses Vol. 13 No. 2 April 1972; and of G. W. Morey and N. L. Bowen, “Corner of system Na2O—CaO—SiO2” published by the Soc. Glass Technol., 9 pp. 232, 233 (1925).
Said patent comment that, the those that it is looked for, is to saturate the sodium, calcium and in general all the elements of a glass formula that are handled with raw materials containing CO2, to provide the specific molecular glass formula or at least a best approach of the molecular formula, completing the balance by providing silica sand
In this way, it is possible to provide a pre-reacted batch of raw materials for a specific molecular glass formula which:                1. Is greatly stable;        2. Melts faster and better;        3. Does not produce any bubbles due to the decomposition of the CO2 components contained in the traditionally used raw material;        4. Reacts or melts at above 1000° C.        5. Is prepared by heating typical raw materials and reacting them as a solid-solid mixture which is de-carbonated between 840° C. and 870° C.:        6. Allows the possibility of improving the glass quality and/or increasing the production rate and or reducing thermal input as well as reducing temperature conditions in the furnace.        7. Allows the possibility of reducing environmental emissions.        8. Allows the possibility of increasing furnace life and/or reducing the size of the furnace for previously equal throughputs.        
The inventor of the present invention have now found that, if cullet is added in the formulation, in bulk form or forming binders, the following advantages were surprisingly found:                1. The velocity of the calcination process is surprisingly increased.        2. The decarbonization grade of the mixture is increased to the double, that when cullet is not being used.        3. A significative impact in the formation of the desirable cristaline structures is presented, since that its use, permits the formation of, in a great part of, calcium and sodium silicate (Na2CaSi3O9).        
The advantages above cited were tested by means of “X” rays diffraction analysis, which were realized on test samples that were carried out in three campaigns.
The mechanism through which the cullet permits the improvements above mentioned, in the process, is not of all known, but this could be explained due to that a better heat transfer of the cullet toward the reactives that are agglomerated in the briquettes that is being formed with the formulations, which allows a better heat diffusion, permitting the formation of the searched species, accelerating the solid-solid reaction.